1. Field of the Invention
The present invention relates to a method for preventing obstruction in aluminum brass pipes which are used for a flow path for water. In particular, the invention relates to a method for preventing obstruction caused by corrosion of aluminum brass pipes and by adhesion of organisms to the aluminum brass pipes. The aluminum brass pipes are used in an industrial cooling water system.
2. Description of Related Art
Aluminum brass pipes, which have a relatively high resistance to corrosion, are conventionally used as capillaries of condensers in nuclear power plants and thermal power plants. In such condensers, obstruction is still caused by corrosion of capillaries, particularly pitting corrosion thereof, and by adhesion of organisms such as mussels (scientific name: Mytilus coruscum), blue mussels (scientific name: Mytilus edulis), freshwater bivalves such as Limnoperna fortunei (scientific name) and barnacles to aluminum brass pipes. The pitting corrosion of capillaries causes leakage of the water, and the adhesion of organisms to the pipes causes blockage of the pipes and impediment of thermal conduction.
In order to cope with the corrosion of aluminum brass pipes, a method is conventionally proposed and carried out wherein ferrous sulfate is continuously added to the water at a low iron ion concentration of about 0.01 to 0.03 ppm (Thermal Nuclear Electric Power Generation, Vol.24. No.10. pp. 1145 to 1156, 1973). Thereby, the aluminum brass pipes are prevented from corroding by iron ions provided to form a corrosion-resistant coating of iron hydroxide on inside surfaces of the pipes.
On the other hand, to cope with the adhesion of organisms to aluminum brass pipes, a chlorination agent or an organic compound is conventionally added. However, in consideration of the environment, the use thereof is not desired. Accordingly, proposed is a method for preventing adhesion of organisms using hydrogen peroxide or an agent capable of generating hydrogen peroxide (Japanese Patent Publication No.SHO 61(1986)-2439). Hydrogen peroxide is easy to decompose, does not cause a problem such as residual toxicity or accumulated toxicity and therefore is safer.
The inventors of the present invention propose methods for preventing the adhesion of organisms using both hydrogen peroxide and iron ions for the purpose of reducing the concentration of added hydrogen peroxide (Japanese Patent Publications Nos.HEI 2(1990)-47277 and HEI 4(1992)-4038).
These methods exhibit a remarkable so-called synergistic effect of hydrogen peroxide and iron ions. Even when the concentration of hydrogen peroxide is reduced to 0.05 to 3 mg/liter, the adhesion of mussels, blue mussels and the like to aluminum brass pipes can be prevented about 98% or more and the growth thereof can also be prevented. The inventors actually carried out the above inventions using model plants in an nuclear power plant and a thermal power plant. Results showed that when hydrogen peroxide and iron ions are added continuously for 24 hours a day, particularly remarkable effect was observed in preventing the adhesion and the growth of organisms.
However, the inventors found that the corrosion-resistant coating of iron hydroxide was not formed on the inside surface of aluminum brass pipes used as capillaries of condensers and that the aluminum brass pipes were corroded, which often results in pitting corrosion. It is considered that hydrogen peroxide inhibit development of the corrosion-resistant coating from iron ions on the inside surface of the aluminum brass pipes.
Then, the inventors has repeated studies and experiments on relation of the addition amount of hydrogen peroxide and iron ions and an adding time period to the formation of the corrosion-resistant coating, finally to find that when the addition amount of iron ion is increased, the corrosion-resistant coating is formed and the corrosion of aluminum brass pipes is prevented. However, the formed corrosion-resistant coating has a considerable thickness and therefore the thermal conductivity declines. In addition to that, because of iron hydroxide particles in the water, the water itself turns brown or bubbles produced at a water outlet are colored.
On the other hand, it is found that, if the addition amount of hydrogen peroxide is decreased within a range which ensures the prevention of the adhesion of organisms to aluminum brass pipes, the corrosion-resistant coating is not formed and therefore the pitting corrosion of pipes cannot be prevented.